Solbian solar panels installed on canvas
An increasingly popular question we hear at Boat Shows is "What size solar panel do I need to run the refrigeration on my boat?" Of course, this begs the question "What is your refrigeration's current draw?" which in itself may not have a set answer.

The current consumption of any refrigeration system is meaningless unless all the conditions are specified, i.e. box temperature, ambient temperature, water temperature, compressor speed, voltage, etc. The manufacturer's figures are simply average numbers; some manufacturers, like Frigoboat, attempt to give a true average figure, while others use their figures more as a marketing tool.

Making use of variable compressor speed, as Frigoboat does with the Merlin and Guardian speed controls, and others do with the Danfoss/Secop AEO control module, brings even higher efficiency and lower overall power consumption in air- and keel-cooled systems, but is counter-productive in a pumped-water system. This is because the pump adds 25% to 35% extra current draw, and so it is best to run the compressor at full speed and get the job done as quickly as possible. The most efficient system is one using the Keel Cooler; no pump, no fan, and variable compressor speed can be used to gain even higher efficiency.

Daily consumption is measured in amp/hours per day, and you can get an idea of what to expect by using a watts meter, like Watts Up, on your refrigerator leads. These meters can tell you how many amp/hours per day a device is using, and it's best to use it over several days to get a daily average. The running consumption will vary dependent on conditions, as will the on/off cycle time, so you will need to be able to come up with an average daily amp/hr draw.

As a general rule of thumb, a solar panel with SunPower® cells will give approximately 1/3 of its rated wattage as a daily yield in amp/hrs. A panel with regular monocrystalline cells will produce about 1/4 of its wattage as daily amp/hrs, and a polycrystalline panel produces around 1/5 of its wattage rating in amp/hrs per day.

Let's take an example:

Monocrystalline; Polycrystalline; SunPower®

Which type of Solar Panel is best for Marine Installations?

3 - 4 - 5 Remember these numbers. These are the factors by which you need to divide the rated wattage of a panel to give an estimate of the amp/hr daily yield, at 12v, that a panel might produce dependent on the type of cell.

    • For a panel with SunPower® cells, divide the watt rating by 3.
    • For a panel with regular monocrystalline silicon cells, divide the watt rating by 4.
    • For panels with polycrystalline cells, divide the watt rating by 5.

Example: If we have one of each type of panel, all rated at 100 watts, then the one with SunPower® cells will produce somewhere in the region of 33 amp/hrs per day; the one with regular mono cells will produce around 25 amp/hrs a day; and the panel with poly cells will produce 20 amp/hrs or so a day. (Note: This is assuming a good solar day, using a MPPT controller, and with a full time load). So why the difference?

While the claim may be that this type of panel has superior shade resistance, the results can be very different.

smaller cell size
: The smaller the cell size, the less shading it requires to be 100% shaded. The same amount of shade that completely covers a small cell might only cover ¼ of a larger cell. Larger cells are better for shade resistance.

solar cells shaded
: If just one cell is 100% hard shaded, the output from that series string of cells will be zero, or close to zero.

: Silicon solar cells consume power as well as produce power. If a cell is shaded, it will consume power from the other cells in the series string, resulting in diminished output and heating up of the shaded cell.

solar panels in parallel
: If several series strings of cells are connected in parallel without the installation of blocking diodes, the power from good strings will feed into a shaded string resulting in diminished panel output and the chance of cells burning.

blocking diodes
: Blocking diodes prevent back-feeding in parallel circuits, but reduce voltage output by 0.7v.

: By-Pass diodes prevent cell burning ("Hot-spots") and do not consume any power or diminish panel output.

: The more cells and electrical connections there are on a panel, the more output-reducing series and shunt resistance the panel will exhibit, and the more chance of a connection failure, particularly if the panel is flexed.

solar cells in series
For best results, look for a panel with large, high efficiency cells, in one series string, with by-pass diodes protecting any string of 50w or greater.

blocking diode drawing
Blocking diodes must be installed on circuits or panels connected in parallel.

© 2018 Coastal Climate Control - All Rights Reserved | ADMIN